1. Field of the Invention
The present invention relates to a manufacturing method with which a magnetic disk having a high coercive force can be obtained.
2. Description of the Prior Art
The advancement of a magnetic disk for higher capacity and of a recording medium (magnetic layer) for higher recording density has been contrived to correspond to the increase in information quantity as is generally known. It is necessary to make the thickness of a recording medium thin to increase the recording density. From this point of view, in place of a coating disk which is considered to have a limit in decreasing the medium film thickness, a plating type magnetic disk having a constitution in which a metallic magnetic layer is formed on a substrate (disk substrate) with a plating method and a sputtered type magnetic disk having a constitution in which a metallic magnetic layer is formed on a substrate with a sputtering method are beginning to be paid attention and some of them are actually used.
It is an effective means to increase the coercive force of a magnetic layer for improving the recording density. In a sputtered type magnetic disk, a method is known to the public as a method to increase the coercive force, in which a magnetic layer being composed of a Co alloy such as Co-Ni-Cr or Co-Cr-Ta is formed with a sputtering method on a substrate in a state of higher temperature than the ordinary substrate temperature of 150.degree. to 250.degree. C. (Summary Notes of Lectures at 11th Meeting of the Applied Magnetic Society of Japan, 1 pA-10, p. 18, 1987, Ishikawa and others, for example). The method is an excellent one, and in mass production, however, there is a problem in an apparatus that a carrier for supporting a substrate can be heat-deformed because a magnetic layer is formed at a high temperature.
Because of this, the present inventors offered a method with which a magnetic disk of an improved coercive force can be obtained by a heating process at a temperature higher than 250.degree. C. after the successive formation of an under layer of Cr, a magnetic layer of a Co alloy, and a protective layer on a carbon substrate being composed of vitrified carbon (Japanese Patent, laid-open number, HER 2-73924; Preliminary Lecture Notes for Spring Meeting of Applied Physical Society, 29a-Y-8, p. 60, 1990). In this case, a similar method is proposed about a magnetic disk having a constitution in which no Cr under layer is provided.
In the above-mentioned manufacturing method of a magnetic disk, it is considered that the coercive force increases with an operating mechanism as described below.
When a non-magnetic element, Cr, is contained in a magnetic layer of a Co alloy, the segregation of Cr into the crystalline grain boundaries is expedited by a heating process. Because of this, the magnetic separation between crystalline grains in the magnetic layer of a Co alloy is expedited, and magnetic interaction between crystalline grains is weakened, which is considered to be the cause of the increase of a coercive force. In a case where a Cr under layer is formed on a substrate, the surface of the Cr under layer (110) makes selective growth in making the thickness of the layer thicker, and the axis of easy magnetization (c axis) of a magnetic layer of a Co alloy can be easily oriented in the surface plane, and the diffusion of Cr into the crystalline grain boundaries in the magnetic layer of a Co alloy from the Cr under layer is expedited by the heating process, which is considered to be the cause of the increase of a coercive force.